Analog echo suppressors utilizing voice switching techniques have long been used in telephone systems to improve the quality of long-distance calls for which the two-way signal delay is such as to result in annoying echos being propagated around the loop. More recently, with the advent of digital transmission, it has been necessary to develop digital echo suppressors to perform the same function. An example of this is found in U.S. Pat. No. 3,673,355 entitled "Common Control Digital Echo Suppression" issued June 27, 1972 and invented by R. E. LaMarche et al. Here, digitized signals representative of the peak amplitude of each of the transmit and receive signals are separately stored. The stored signals are periodically updated as the magnitude of the peak signals increases or decreases. A periodic comparison is made to determine the relative magnitude of the digitized signals. Upon detection that the received signal exceeds a certain threshold, digital attenuation of the transmit signal is effected to provide echo suppression. Additional circuitry is also provided to insert sufficient attenuation in the receive path during a double talking condition to reduce the echos to a tolerable level.
In a two-wire circuit for analog transmission of a local telephone call, echo suppression is not necessary since the signal delay is relatively short and no major impedance mismatch occurs which would give rise to signal reflections. With digital switching of local calls, the analog signals from the station set are converted to a digital format prior to switching in a central office (CO) or a private branch exchange (PBX). However, digital signals cannot be readily transmitted and received on a two-wire line. Hence, the analog signals from the telephone set are initially split prior to encoding and decoding, into a four-wire circuit utilizing a so-called hybrid network. Because it is not economically feasible to prevent all leakage across the hybrid network, some of the received signal from the decoder (D/A converter) is coupled through the hybrid network to the encoder (A/D converter) in the transmit path. If the phase delay around the four-wire path is such that the leakage signal reinforces or cancels the incoming signal from the telephone set at some frequency in the frequency range transmitted, severe distortion and even oscillation can result.
One possible solution to this dilemma is to insert fixed attenuation in either the digital or analog portions of the four-wire circuit. While such a solution is acceptable for long distance or toll calls, it is not tolerated in a local call. Thus, it is necessary that substantially zero loss be encountered over the talking path of two telephones on a local call. Hence the only practical solution is to insert sufficient attenuation in the listening path to substantially negate any severe distortion or oscillation. Such attenuation can be inserted in the listening path at either the transmit or receive ends and must be switched in and out in response to the relative signal levels of the two paths. This can be done in either the digital or analog portions of the four-wire path. However because the attenuation must be applied to each four-wire circuit, prior circuits such as discussed above for providing echo suppression in long distance calls encountering lengthy transmission delays, are not economically feasible for application to each local call due to the complexity of the circuitry required to control the attenuators.